Understanding and enhancing the phase stability of fast pyrolysis oils through ternary phase diagrams

• FPO phase stability is determined by the fractions of PL, MS, and water. • Phase separations consistently occurred at 25 °C when PL content exceeded 56 wt%. • Phase separations consistently occurred at 25 °C when MS content was below 28 wt%. • Solvents with more –OH groups were less effective in enhancing stability. • For mono-alcohols, a lower ratio of –OH groups was more favorable. Fast pyrolysis oils (FPOs), derived from various biomass feedstocks, hold promise as carbon–neutral fuels. However, their industrial use is hindered due to phase separations during storage, transportation, and co-processing, with limited understanding of the root causes that drive this phenomenon. This study aims to elucidate the kinetic stability of FPO by developing ternary phase diagrams and exploring the effects of the temperature and addition of organic solvents on the aging. The phase stability of FPO was found to be determined by the fraction of pyrolytic lignin (PL), mixed solvent (MS), and water. Even at 25 °C, phase separations could consistently occur when the PL content exceeded 56 wt% or the MS content fell below 28 wt%. Heating the FPO to 50 °C–80 °C led to a decrease in the water-soluble content and an increase in the water-insoluble content, while the water content remained relatively unchanged, indicating the transition of the water-soluble content to the water-insoluble content was closely related to FPO phase separation during aging at elevated temperatures. Moreover, the solvents could slow the aging of the FPO in the order of 2-propanol > ethanol > methanol > ethylene glycol > glycerol. Thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR-spectroscopy) analysis further revealed changes in both the chemical compositions and polymerizations during aging, in which alcohols were identified as effective inhibiters to slow the aging. These new findings provide promising pathways to controlling the FPO phase stability through solvent selections and will advance the adoptions of FPOs.